1. Field of the Invention
The present invention relates to a valve timing control device for changing the valve timing of an engine and controlling the valve overlap amount between the intake valve and the exhaust valve of the engine in accordance with the operating conditions of the engine. More in particular, the present invention relates to a valve timing control device for an engine which is capable of reducing the emission without deteriorating the engine fuel consumption rate by correcting the valve timing even in the case where a fuel of a different property is supplied.
2. Description of the Related Art
Conventionally, the timing of operating the intake valve mounted on the cylinder head of the engine and the exhaust valve is uniquely synchronized with the rotational phase of the crankshaft, and therefore, the amount of the intake air and the exhaust air in the combustion chamber is dependent on the opening degree of the throttle valve arranged in the intake manifold and the rotational speed of the engine.
In recent years, however, a valve timing control device (variable valve timing mechanism VVT) has found a practical application, in which the timing of operation of the intake valve and the exhaust valve (hereinafter sometimes referred to as the intake and exhaust valves) can be changed so that the amount of intake and exhaust gases in the combustion chamber can be adjusted with a higher degree of freedom. This valve timing control device comprises a mechanical variable mechanism rendering the timing of the operation of the intake and exhaust valves variable and a control unit using a computer for controlling the operation of the variable mechanism. The control unit controls the variable mechanism in accordance with the operating conditions of the engine. Thus, the operation timing of at least one of the intake valve and the exhaust valve is controlled thereby to control the length of the period during which both the intake valve and the exhaust valve are open (the magnitude of the valve overlap amount). As a result of this control operation, the amount of air introduced into the combustion chamber is adjusted to proper amount, or the exhaust gas that has been burnt in the combustion chamber (burnt gas) and returns to and stay in the combustion chamber, i.e. the internal EGR amount is maintained in proper amount for an improved engine output, emission and fuel consumption.
An example of this type of valve timing control device is described in the Unexamined Patent Publication (Kokai) No. 4-194331.
The device described in this publication switches the valve timing in accordance with the operating conditions of the engine. Specifically, when the engine is running at low speed under a light load, the valve overlap amount is set to a low level. Also, when the engine is running in cold state at low temperature, the overlap amount is set to a small level over the entire operating areas.
Generally, when the valve overlap amount is set to a large level, both the intake valve and the exhaust valve remain open for a longer period during the exhaust stroke, and the burnt gas in the cylinders is liable to return to the intake port in a phenomenon called "blow back". Especially when the engine is running under a light load with a small throttle valve opening degree and a high negative pressure in the intake port (that is to say, a low absolute pressure in the intake port), a valve overlap is liable to cause blowing back of the burnt gas.
A large amount of the burnt gas blew back into the intake port causes the burnt gas returned to the intake port to be introduced again into the cylinder during the intake stroke. This poses the problem of a reduced amount of new gas supplied to the cylinder (an increased internal EGR amount). Especially when the engine temperature is low, an increased amount of the fuel supplied to the intake port attaches to the wall surface of the intake port (a wall-attached fuel) with the result that a sufficient amount of fuel is not supplied to the cylinder.
Specifically, when the engine temperature is low, the fuel supplied to the intake port is not easily vaporized, and therefore forms comparatively large liquid fuel particles in the intake port. If the burnt gas returns to the intake port under this condition, the fuel particles in the intake port are blown back and attach to the wall surface thereof. Also, the lower the engine temperature, i.e. the lower the efficiency of fuel vaporization, the larger the amount of the fuel that is blown back and attaches to wall surface of the intake port. If the spitting of the burnt gas occurs when the engine temperature is low, therefore, the fuel attached to the wall surface of the intake port prevents a sufficient amount of fuel from being supplied to the cylinder, thereby causing a cold hesitation (called the delayed acceleration at low temperature) or posing the problem of unstable combustion due to the increased internal EGR amount.
The Unexamined Patent Publication (Kokai) No. 4-194331 described above discloses a method in which, when the engine is running at low speed under a light load and liable to cause the spitting due to a high negative pressure of the intake port, the valve overlap amount is reduced to reduce the spitting of the burnt gas to prevent an increase of the internal EGR amount and the deterioration of the fuel combustion. Further, when the engine temperature is low, the valve overlap amount is reduced over the entire operation range thereby to reduce the amount of fuel attaching to the wall surface due to the spitting of the burnt gas.
As described above, in the device described in the aforementioned patent publication, the valve overlap amount is set to a lower value when the engine temperature is low than when the engine temperature is high in order to stabilize the combustion by reducing the internal EGR amount and to prevent the cold hesitation by reducing the amount of fuel attaching to the wall surface of the intake port.
Another problem is that when the engine temperature is low, the operating speed of the valve timing control device for changing the valve timing decreases to such an extent that the engine may misfire depending on the change in the operating conditions. Specifically, when the engine temperature is low, the increased friction in the various parts and the increased viscosity of the engine oil decreases the operating speed of the valve timing control device. As a result, when the engine temperature is low, it takes longer to change the valve overlap amount to a level suitable for the prevailing operating conditions, and a difference occurs temporarily between the optimum valve overlap amount and the actual valve overlap amount. In the case where the actual valve overlap amount is smaller than the optimum value, the engine output decreases but no serious operating problem is posed. In the case where the valve overlap amount is excessively large as compared with the optimum value, however, a misfire may occur. In an extreme case, the engine becomes impossible to operate. In view of this, when the engine temperature is low, the valve overlap amount is set normally to a level lower than when the engine temperature is high. In this way, the actual valve overlap amount is prevented from increasing excessively as compared with the optimum value even when the operating speed of the valve timing control device decreases, thereby preventing a misfire.
Specifically, in the prior art, when the engine temperature is low, the valve overlap amount is set to a value smaller than when the engine temperature is high, in order to (1) reduce the internal EGR, (2) to reduce the amount of fuel attached to the wall surface of the intake port, and (3) to prevent a misfire which otherwise might be caused by the slower operating speed of the valve timing control device.
If the valve overlap amount is uniformly decreased by a valve timing control device like the one disclosed in the above-mentioned patent publication, in which the valve overlap amount of the cylinder is changed by changing the valve timing, however, the engine performance is considerably reduced when the temperature is low. In the case where the valve overlap amount is changed by changing the valve timing, the valve open period is normally maintained constant. For example, in a case in which the valve overlap amount is controlled by changing the operation timing of the intake valve, the earlier (advanced) the operation timing of the intake valve, the more the valve overlap amount increases, while the later (retarded) the operation timing of the intake valve, the more the valve overlap amount decreases. In the case where the valve overlap amount is changed by controlling the valve timing, therefore, a small setting of the valve overlap amount delays the close timing of the intake valve at the same time so that the intake valve undesirably closes during the compression stroke after the intake stroke of the cylinder. In this way, once the close timing of the intake valve overlaps with the compression stroke of the cylinder, the gas that has already been introduced into the cylinder is pushed back to the intake port from the intake valve during the compression stroke, resulting in a lower intake gas volume efficiency of the cylinder. Especially when the engine is running at low speed, the supercharge effect due to the intake inertia cannot substantially be secured and therefore, a delayed close timing of the intake valve considerably reduces the engine output due to a reduced intake air volume efficiency.
On the other hand, the fuel attached to the wall surface of the intake port, for example, even in the case where the burnt gas is somewhat blown back due to the valve overlap when the engine temperature is low, is not easily attached to the wall surface if the fuel is vaporized efficiently. Even when the engine temperature is low, therefore, it should be possible to increase the valve overlap amount in the case where a light fuel high in vaporization efficiency is used unlike in the case where a heavy fuel, not easily vaporized, is used.
Also, in the case where the friction is small in the valve timing control device or a working oil low in viscosity is used, a misfire should be prevented even if the valve overlap is set to a large amount at a low temperature when the operating speed of the valve timing control device is high unlike when the operating speed is low.
For this reason, if the valve overlap amount is set uniformly to a small value when the engine temperature is low as in the device disclosed in the Unexamined Patent Publication (Kokai) No. 4-194331 described above, the unnecessary small setting of the valve overlap amount may lead to the operation with a lower engine output depending on the operating conditions.
Some valve timing control devices comprise a variable valve timing mechanism capable of changing the operation timing of the intake and exhaust valves continuously without switching between small and large values. In this variable valve timing mechanism of continuous variable type, the engine operating conditions are detected by a computer thereby to calculate a target operation timing of the intake and exhaust valves, and at the same time, the actual operation timing of the intake and exhaust valves is detected, so that the variable timing mechanism is controlled by feedback in such a manner that the target operation timing and the actual operation timing of the intake and exhaust valves coincide with each other.
In the case where this variable valve timing mechanism of a continuously variable type develops a trouble such that the variable mechanism fails to operate as intended, the engine suffers from various faults. Assume, for example, that the variable valve timing mechanism fails to operate with a large valve overlap amount under a light engine load. The engine may be stalled. Once the engine is stalled, any subsequent trouble in the variable valve timing mechanism cannot be detected.
In view of this, a fault detection device for the engine valve timing control device has been proposed (see the Unexamined Patent Publication (Kokai) No. 9-60535), in which a trouble of the variable valve timing mechanism of continuous variable type can be detected even after the engine comes to stop due to a stall by detecting the trouble of the control device based on the judgment of the occurrence of an engine stall.
The variable valve timing mechanism of a continuously variable type disclosed in the Unexamined Patent Publication (Kokai) No. 9-60535 described above has the effect of reducing NOx due to an increased internal EGR and improving the fuel consumption due to a decreased pumping loss by setting the valve overlap amount to a large value.
The components of the fuel used for the engine, however, are not constant under all operating conditions, but are divided into several types according to the properties to assure the same volatility in accordance with the temperature of the location where the fuel is mainly used. The fuel properties are roughly classified into three types described below.
Light fuel: Fuel having many components of low boiling point
Heavy fuel: Fuel having few components of low boiling point
Intermediate fuel: Fuel having components of a boiling point intermediate between the light and heavy fuels
The variable valve timing mechanism described in the Unexamined Patent Publication No. 9-60535, however, fails to take the aforementioned fuel properties into consideration, and fails to perform the control operation in accordance with the fuel properties. In the case where a heavy fuel is used, for example, more fuel is attached to the wall surface. The fluctuation of the amount of the heavy fuel thus attached may destabilize the combustion.